The first plate-girder, or tubular, bridg,e was erected in England in 1846 by Stephenson on the line of the North-western Railway. The lattice-girder and open-truss bridges constitute a later development of the art.
The Roadway of iron bridges is formed of a series of cross-beams on which are placed longitudinal pieces, and on these, in turn, rest an upper series of cross-ties or beams to receive the railway-track or the road-surface. To stiffen the bridge-structure laterally, so that it may sustain stresses due to violent storms of wind, some system of bracing is adopted, of which illustrations are shown on Plate 43 (figs. If the roadway is placed on the top chord, the structure is termed a top-road (deck) bridge (pi. 45, figs. 1, 2); if on the bottom chord, it is called a bottom-road (through) bridge (fii. 43, fig. 1o).
E.rpansion and Contraction of Iron Bridges.--lnasinuch as, in conse quence of changes of temperature, the length of the bridge frame must vary, provision must be made to permit of some motion of the structure in the direction of the bridge axis. For this piirpose it is customary to secure the bridge girder upon its pier or abutment at one end only, leaving the other end to rest upon a so-called " roller-bearing," upon Nvhich it may move to and fro to accommodate itself to the changes of temperature. The weight of the free end of the girder in this case is borne on hardened steel rollers, which afford the least possible resistance to its movements. For transmitting. the distribution of the pressure equally to all the rollers, the " rocking-bearing " shown in Figure 16 (pi. 43) has proved a very service able device. In this, the end of the girder rests upon the rollers through the intervention of a curved bearing-plate.
Construction of Girder Bridges.—The usual practice is to construct iron bridges with each span independent, although continuous-girder bridges are not uncommon. The coutinuous-girder bridge affords some saying of material and can be erected without the use of staging; it is put together on the land and bodily pushed out into position on its piers, as was done in the cases of the Stadtlau Bridge over the Dannbe Canal at Vienna, and the Seryian Raib.yay Viaduct at Freiburg. The success of this plan, however, will depend largely upon the unalterabilitv of the height of the piers. Should a slight sinking take place in one of these, it might seriously affect the stability of the bridge unless special provisions were made for compensating for the irregularity.
The American 5),stem claims several decided advantages—namely, it permits of the preliminary assemblage of the parts in the shop, where any defect may be observed and corrected; by the use of machines of improved construction, the lengths of the various members, the drilling of the holes for the pins, and the dimensions of the pins are controlled with the great est possible accuracy, so that the erection of the structure may be accom plished rapidly and without demanding specially-skilled labor; the proba bility that the strength of the completed bridge will vary from that calcu lated for it must necessarily be much less than where, as in the European plan of erection first at the place of construction, there is no opportunity of determining, by preliminary trial at the shops, the existence of errors in calculation or of hidden defects; and filially, with the same strength, the American system of construction allows of a much lighter structure, repre senting economy of material and cost. The several advantages possessed
by the American system of iron-bridge construction, as compared N.N.ith that followed in Europe, have of late been freely recognized by the most eminent European authorities.' No/able Gira'er the notable wrought-iron girder bridges of the world the following may be named: The Ohio River Bridge of the Cincinnati Southern Railway at Cincinnati, a single span of 515 feet; the Lek River Bridge at Kuilenburg-, Holland (I87o), with one span of 492 feet, one of 262.5 feet, and seven of 187 feet; the Tamar Bridge at Saltash, with two spans of 455 feet and seventeen smaller spans; the Ohio River Bridge at Cincinnati, on the Newport and Cincinnati Railroad, with a span of 42o feet; the Ohio River Bridge at Louisville (IS6S), 5318.8 feet long and with a inaximum span of 400 feet; the bridge over the Missouri River near Kansas City, with three spans of 400 feet each, one of 25o, one of 2oo, and two of.175 feet each, and a viaduct 2000 feet long; the Vistula Bridge at Dirschau (1857), with six spans of 397 feet (pl. 43, jig-. 12); the Waal Bridge at Bominel, with three spans of 393.7 feet and eight of 187 feet; the Kentucky River Bridge at Dixville, with three spans of 375 feet each; the Parkersburg and Bellaire Bridge over the Ohio for the Baltimore and Ohio Railroad, with spans of 35o feet; the Rhine Bridge (1862) at May ence (fig. 17), with four spans of 345 feet and twenty-eight smaller spans; the Rhine Bridge at Hamm (r87o), with four spans of 34o feet; the Rhine Bridge at Griethausen, with one span of 329.4 feet and twenty smaller spans; the bridge over HolIands-Diep at Moerdyk (1871), with fourteen spans of 328 feet each and two smaller spans; the Rhine Bridge at Cologne (186o), with four spans of 322 feet; the St. Charles Bridge over the Mis souri, with channel spans of 321.5 feet; the Ohio Bridge at Steubenville, with main span of 319 feet; the Elbe Bridge at Hamburg (IS7o), with three spans of 316 feet; the Ohio Bridg,e at Bellwood, with main span of 305 feet; the Wye Bridge at Chepstow (1852), with one span of 3oo feet and three smaller spans; the Rhine Bridge between Mannheim and Ludwigshafen (1S67), with three spans of 295 feet; the Boyne Bridge at Drogheda (1S55), with one span of 267 feet and two of 14o feet each; the Danube Bridge of the Austrian North-western Railway at Vienna (1S72), with four spans of 252 feet and fourteen of 97 feet; the Danube Canal Bridge at Vienna (1869), 25o feet; the 'Mississippi Bridge at Keokuk-, Iowa, with main spans of 250 feet; the Danube I3rich-,re Stadtlau (187o), with five spans of 249 feet and ten of to feet; the Kinzig Bridge 0858) at Offenburg 43,.figs. to, t), with a single span of 197 feet, etc.